(a) Field of the Invention
The present invention relates to a heat sensitive recording matrerial. More particularly, it relates to a heat sensitive recording material which has an excellent preservability before it is used for recording and a high color density at the time of thermal recording.
(b) Prior Art Description
Recently, a thermal recording method is rapidly spreading in such fields as facsimile and printers due to the following merits, although a heat sensitive paper used for this purpose often has a poor preservability before and after recording.
(1) Development is not needed. PA1 (2) If the support is paper, its quality is nearly the same as that of ordinary paper. PA1 (3) The material is easy to handle. PA1 (4) It has a high color density. PA1 (5) The recording unit used is simple and inexpensive. PA1 (6) No noise is caused on recording. PA1 paper described in Japanese Patent Application (OPI) No. 136492/83 and having an optical surface roughness of not more than 8.mu. and a thickness of 40-75.mu. PA1 paper described in Japanese Patent Application (OPI) No. 69091/83 and having a density of not more than 0.9 g/cm.sup.3 and an optical contact rate of not less than 15% PA1 paper described in Japanese Patent Application (OPI) No. 69097/83, manufactured from the pulp which was beaten so as to make its Canadian Standard Filtration (JIS P8121) not less than 400 cc, and treated to prevent the permeation of the coating solution PA1 paper described in Japanese Patent Application (OPI) No. 65695/83 which is prepared by coating the glossy surface of base paper manufactured by Yankee Machine and has improved coloring density and resolving power PA1 paper described in Japanese Patent Application (OPI) No. 35985/84 which is prepared by corona treatment of base paper and has improved the characteristics for coating
As a heat sensitive recording material, heat sensitive paper which can be used for multi-coloring has been developed in recent years. For example, it includes the one utilizing diazosulfonate or a combination of diazosulfonate and leuco coloring system. However, in the case of this type of paper, it is necessary to differentiate the thermal reactivity by selecting the compounds which conduct coloring reactions. Therefore, it has been inevitable that the coloring materials which could be used for such a purpose were highly limited in number, resulting in a heavy restriction on designing and manufacturing recording materials.
Also, though the heat sensitization recording has been so far mostly applied to literal recordings, there is increasing a demand for picture recordings with gradation such as hard copies of television pictures. In this case, gradation is expressed either by setting the recording density at a certain level and altering the recording area or by setting the recording area to a certain size and altering the recording density. Since the former not only deteriorates the resolving power but also has a limitation on expressing half-tone, the latter is an excellent method.
However, although usual heat sensitive paper is usable for the former method, it is not suitable for the latter which needs printing of an intermediate density. In other words, it has been difficult to supply heat sensitive recording materials which have a reasonable changing rate of the coloring density in response to an applied energy so that the coloring density can change gradually with a certain smoothness.
For example, in the case of heat sensitive recording paper of a leuco coloring system, it is prepared by finely dispersing a leuco dye and a phenolic developer in a size of microns, mixing and applying the components to a paper support. When this paper is heated with a thermal head, the above-mentioned coloring components fuse and mix to form color. Therefore, it is necessary to differentiate the temperature characteristics of each kind of particles or layered coatings on paper in order to let the coloring density change in response to the applied energy. This is not an easy procedure.
As a method for solving the above problems, we have already proposed a heat sensitive recording material which has an improved preservability (Japanese Patent Application (OPI) No. 190886/84 (the term `OPI` as used herein means an "unexamined published application")). In this method, at least one component which relates to a coloring reaction is enclosed as a core substance and a wall is prepared around the core by a polymerization reaction to form a microcapsule. The capsules are contained in a recording layer.
We improved this method further and proposed a heat sensitive recording material and/or a multi-colored heat sensitive recording material which can reproduce a half-tone. This is accomplished by combining various microcapsules of which wall materials are varied so as to give them different thermal responses and, as a result, a picture density can be varied (Japanese Patent Application Nos. 89781/84 and 99489/84). However, there was a weak point in such a material using microcapsules because it was difficult to secure a sufficiently high thermal coloring density.